Richard Scher, M.D., professor of clinical dermatology for the College of Physicians and Surgeons, Columbia University, New York.

Almost 30 million Americans—approximately 13% of the population—has a fungal infection of the nails (also known as onychomycosis.)

As a percentage of salon clients, though, Richard Scher, M.D., estimates the percentage drops to less than 5%. Even so, questions on onychomycosis have topped nail techs' lists of questions on nail health for NAILS' medical experts over the past decade.

Your questions prompted us to assemble the ultimate nail tech's guide to onychomycosis.

We compiled your most frequently asked questions, added a few of our own, and posed them to seven dermatologists around the country who research, teach, and treat onychomycosis every day. When there was a difference of opinion or additional information, we provided responses from two or more doctors.

If you have more questions about onychomycosis after reading this, send them on. Our experts are standing by.

What are the classic signs and symptoms of onychomycosis?

Dr. Abuabara: The most common changes are a yellow or whitish discoloration of the nail (a long­standing infection may turn the nail brown or black), thickening of the nail, separation of the nail plate from the nail bed, brittle nails, less commonly, redness around the infected nail and loss of the nail.

Dr. Gold: A whitish discoloration usually signifies a yeast infection, while a yellow or brown discoloration usually indicates a dermatophyte infection. Both yeast and dermatophytes are types of fungi. [See "Fungus Unplugged" for a more detailed discussion of these fungi.] However, other organisms, including several bacterial agents, can affect the nails. For example, green indicates a pseudomonas (bacterial) infection, while a black discoloration signifies a proteus infection. If you see what appears to be a superficial white discoloration or yellow/brown discoloration underneath the nail, refuse the service and refer your client to a medical professional knowledgeable about nail diseases, such as a dermatologist.

How and where are most fungal infec­tions of the skin and nails contracted—is the old saw about wearing shoes in public showers really true?

Dr. Lynde: Fungal spores are all around us in the environment. Fungi especially thrive in moist places, so the old saw about gyms and public showers is probably true. But you're equally at risk if someone in your home has a fungal infection because the fungi are in the shower, the carpet, etc.

Dr. Daniel: Developing onychomycosis is usually a long-lasting process where the person has athlete's foot for years then traumatizes the nail, etc. I have, however, had patients who suggested they had gotten other types of infections in a salon.

How—and why—do fungi infect the nails?

Dr.Lynde: Some types of fungi feed on keratin, the dead skin cells that comprise nails. But fungi can't infect healthy nails that have a good vascular supply because fungi also need a warm, moist environment to survive. Some type of trauma usually precedes fungal infections of the nails, whether a person stubs her toe or wears poor-fitting shoes. It often takes four to five months for onychomycosis to become apparent, though, and many people have forgotten the original injury by then.

Why is onychomycosis on the increase in the United States?

Dr. Daniel: People are living longer and are more mobile. Onychomycosis is more common in older individuals: Studies show that less than 1% of pre-teens have onychomycosis. Between 5% and 10% of individuals up to age 30 have onychomycosis, and only 20% of people ages 30-60. In people over age 60, the number jumps to 40%-50%.

People are not only living longer, they're exercising more. Exercise can traumatize the nail and break the seal between the nail plate and nail bed. Studies show that almost 100% of the time the fungal infection starts on the bottom of the feet, moving into the nail unit when that seal is broken.

Who is at risk for onychomycosis?

Dr. Abuabara: The risk factors include diabetes, debilitating disease, or a compromised immune system such as that found in people with AIDS. Other risk factors include poor circulation such as caused by peripheral vascular disease. Also, trauma creates a portal for the fungi to invade the undersurface of the nail. Finally, poor nutrition and hygiene and repeated exposure to infected persons, animals, or surfaces (such as the floor at the local gym) all play a role.

Why are fungal infections found so much more commonly on the toenails than fingernails?

Dr. Rich: True fungal infections of the fingernails are about one-tenth as common as on the toenails. The toenails are exposed to much more trauma and abuse than the fingernails. The feet also are more susceptible to fungal infections of the skin because they spend so much time encased in socks and shoes, which foster the dark, moist environment fungi thrive in.

Can nail techs safely service clients with a fungal infection?

Dr. Lynde: As long as you sterilize or dispose of your implements between clients, you can safely service those with fungal infections without putting others at risk.

As for the infected nail, you can safely file it down, carefully clip it back, and apply polish. Do not apply artificial nails.

Trimming back the nail removes some of the subungual debris—the whitish-yellowish, crumbly material. It also helps prevent clients from catching the separated edge of the nail, which not only hurts, but also can help the fungi spread. Don't, however, dig under the nail to get at this debris. Digging can cause the nail plate to separate from the bed and allow the fungi to move further back under the nail.

Nail polish enhances the living conditions for fungi, but I tell my patients that it takes months for the medicine to work, so it's OK to conceal the condition in the meantime.